Mott and Efros-Shklovskii Variable Range Hopping in CdSe Quantum Dots Films

The model of variable range hopping conductivity predicts a crossover between Mott and Efros-Shklovskii as a function of temperature and density of states. This is observed using monodispersed CdSe colloidal quantum dot 3D solids where the density of states at the Fermi level is varied by electrochemistry. At low density of states, both below the lowest state (<0.4e(-)/dot) and in the conductivity gap between the first and second state (2e(-)/dot), the temperature dependence of the conductivity shows the 1/4 exponent of Mott hopping. At other fillings up to 6e(-)/dot, the conductivity shows the 1/2 exponent of Efros-Shklovskii hopping. The non-Ohmic conductivity is also found to be explained quantitatively by the variable range hopping model.